The present disclosure relates to an electrolyte solution, a method for producing an electrolyte solution, and an electrochemical device. More specifically, the present disclosure relates to a suitable electrolyte solution, and method for producing that electrolyte solution, that is used as the electrolyte layer in a magnesium (Mg) ion battery for example, and various electrochemical devices having a magnesium ion battery and the like that uses this electrolyte solution.
Magnesium ion batteries are drawing attention as a next-generation secondary battery to replace lithium ion batteries due to the fact that compared with lithium, magnesium is a much more abundant natural resource and is far cheaper, the amount of electricity per unit volume that can be extracted by a redox reaction is large, and there is also a high level of safety when used in a battery.
In the past, magnesium electrolyte solutions for a magnesium ion battery have all used an ether solvent. Especially, it has been reported that the electrolyte solutions that use tetrahydrofuran (THF) have the best properties (refer to JP-T-2003-512704, JP-A-2009-21085, and Nature Communications Volume: 2, Article number: 427 DOI: doi:10.1038/ncomms1435).
However, ether solvents such as THF are difficult to handle due to their high volatility and because they are often toxic. Further, the potential window (the maximum voltage that can be applied without the electrolyte solution degrading) of a magnesium electrolyte solution that uses an ether solvent is at most about 3.0 V, which is small. Consequently, it has not been possible to produce a battery having a high voltage using a magnesium metal negative electrode.
Due to the above reasons, development has proceeded on magnesium electrolyte solutions that use a solvent other than THF. As a result, a magnesium electrolyte solution was discovered that can be used on magnesium alloy (refer to Electrochemistry Communications 16 (2012) 103-106).
JP-A-2003-100347 discloses a non-aqueous electrolyte battery that uses alkyl sulfone, which is a non-ether solvent, for the electrolyte solution. Specifically, this non-aqueous electrolyte battery includes a positive electrode, a negative electrode including at least one element selected from aluminum, calcium, and magnesium, and a non-aqueous electrolyte solution. This non-aqueous electrolyte solution contains an alkyl sulfone represented by R1R2SO2 (wherein R1 and R2 represent an alkyl group) and at least one selected from an aluminum salt, a calcium salt, and a magnesium salt in a mixed solvent of the alkyl sulfone and an organic solvent that dissolves the at least one selected from an aluminum salt, a calcium salt, and a magnesium salt. As the organic solvent, at least one selected from γ-butyrolactone, acetonitrile, and propylene carbonate is used.